What is the difference in caching between HTML5 and the old HTML?

The Cache on the application level is a great feature for the HTML5 where its not found in the old HTML that use the traditional browser cache.

This feature allows you to cache some data (static data) locally and serve your web app offline on a specific PC or Mobile.

For more info about HTML5 application cache you can visit w3 schools tutorial.

Regards, 

An important feature of HTML5 is the Application Cache. It creates an offline version of a web application. and stores website files such as HTML files, CSS, images, and JavaScript, locally. It is a feature that speeds up site performance.

old browsers are dieing :) so use the latest technologies that can gear up your applications/web sites. or die with them... app cache is used for static contents. Image files, css, js etc. they will boost your performance.